1. Field of the Invention
The present invention relates to an additive for masticating natural rubber capable of reducing the amount of gel without lowering the molecular weight and capable of improving the processability as well, a mastication method of natural rubber using the above additive, a natural rubber composition obtained by the above mastication method, and a pneumatic tire using the above natural rubber composition.
2. Description of the Related Art
In rubber products such as tires, belts and hoses, blended are various compounding agents such as vulcanizing agents and reinforcing agents. In order to facilitate this blending work and molding work of the rubber products, a mastication step for crude rubber (natural rubber) which Is a raw material is essential.
In general, the significance of mastication lies in providing crude rubber with satisfactory plasticity by giving shearing force to it. This mastication facilitates blending and milling work (work for milling homogeneously various compounding agents into crude rubber) like dispersing of various compounding agents such as vulcanizing agents and fillers. Further, the mastication stabilizes the dimensions, makes the skin fine and makes scorching difficult in after-steps of rolling and extruding, that is, it serves the purpose for making it easy to carry out these operations.
In this mastication, the molecular weight is also reduced by breaking the molecules of crude rubber to have the rubber plasticized by mere mechanical mastication using a roll machine, an internal mixer and the like. However, such mechanical mastication requires electricity, labor and time in very large quantities.
Accordingly, it is generally known well that a peptizer is used in a mastication step of crude rubber.
Thiophenols having substituents on an aromatic ring and disulfides have so far been used as a peptizer.
However, there are such problems that the use of these peptizers allows the plasticization to go on to reduce the viscosity of the rubber and that at the same time, the molecular chains of the rubber are broken and vulcanized rubber obtained by vulcanizing the rubber becomes inferior in breaking characteristics.
On the other hand, in natural rubber there exist gels formed by intertwined molecular chains and by the reaction between functional groups contained in the isoprene chains of the natural rubber or the reaction of such functional groups with non-rubber components contained in the natural rubber. This gel component causes an increase in viscosity of crude rubber (storage hardening) or poor dispersion of the rubber compounding agents.
Ordinary mastication reduces this gel component but breaks the molecular chains at the same time. Accordingly, the same problems as described above take place.
Thus, so far there have been no techniques for selectively reducing the amount of gel without lowering the molecular weight of natural rubber.